In a number of applications, it is desirable to recognize an object and map its shape and position by observing it with a laser both at a distance and at close proximity. For example, a system with this capability might be used in an airborne vehicle overflying a terrain at very low altitude.
Such recognition and mapping requires an accurate measurement of distance over a range extending from kilometers down to about a decimeter. Because light travels at the rate of about 30 cm/ns, the accurate measurement of such a distance requires operation of the measuring citrcuitry at a frequency in the gigahertz range.
It has been proposed to measure the elapsed time between the transmission and the receipt of a laser pulse in a system of this type by means of a gallium arsenide (GaAs) digital counter. Although such counters are technically feasible, circuit board layout becomes critical for a counter operating in the gigahertz range, and there is a considerable risk of unpredictable malfunctions. In addition, GaAs digital counters are prohibitively expensive for reasonably priced ranging apparatus.
Timing methods involving sampling a ramped analog voltage and digitizing the sample have been used in other technologies. That approach would not only be economically attractive, but it would also overcome the layout and reliability problems of gigahertz-range GaAs counters. However, the use of ramped-voltage timing techniques was traditionally considered too slow to be useful in the gigahertz environment of short-distance laser ranging devices.
Prior art in this field includes U.S. Pat. No. 4,139,301 which describes an input rectifier circuit for a laser distance measuring system; U.S. Pat. No. 4,464,048 which discloses a temperature-compensated and scatter-eliminating receiver for a laser distance measuring device; U.S. Pat. No. 4,538,907 which concerns a laser ranging device involving a comparison of the timing of a direct and a reflected laser pulse; U.S. Pat. No. 4,734,587 which describes a laser distance measuring system using a clock pulse counter; and U.S. Pat. No. 4,669,508 which shows a high-precision laser distance measuring device comparing a target beam with a reference beam whose path length is known.